RF communications systems typically communicate using at least one of three different modes of operation. The first mode, called simplex, is a one-way mode of operation, in which a transmitter from one location transmits data to a receiver at another location. For example, a broadcast radio station transmits data one-way to radios. The second mode, called half duplex, is a two-way mode of operation, in which a first transceiver communicates with a second transceiver; however, only one transceiver transmits at a time. Therefore, the transmitter and receiver in a transceiver do not operate simultaneously. For example, certain telemetry systems operate in a send-then-wait-for-reply manner. The third mode, called full duplex, is a simultaneous two-way mode of operation, in which a first transceiver communicates with a second transceiver, and both transceivers may transmit simultaneously; therefore, the transmitter and receiver in a transceiver must be capable of operating simultaneously. In a full duplex transceiver, signals from the transmitter must not interfere with signals received by the receiver; therefore, transmitted signals are at transmit frequencies that are different from received signals, which are at receive frequencies. The difference between a transmit frequency and a receive frequency is called the duplex frequency. For example, certain cellular telephone systems operate using a full duplex mode of operation.
Full duplex transceivers using a single antenna often use a duplexer to couple the transmitter and receiver to the single antenna. A duplexer enables simultaneous transmission and reception of RF signals by providing a transmit passband that does not overlap with a receive passband, which prevents interference between transmit and receive signals. The non-overlapping area is also known as a duplex gap. Some communications protocols, such as specific Universal Mobile Telecommunications System (UMTS) bands have duplex gaps that are narrow relative to the transmit and receive passbands; therefore, providing the required transmit and receive passbands with minimal insertion loss while providing required isolation between transmit and receive signals may be difficult.
Additionally, as wireless communications technologies evolve, wireless communications systems become increasingly sophisticated. As a result, multi-mode and multi-band wireless systems are becoming routinely available. Such systems may include common circuit elements to support multiple modes, multiple bands, or both to reduce size, cost, and insertion losses. Thus, there is a need for a multi-mode duplexer architecture that supports multi-mode functionality, simplifies front-end architectures, and provides required transmit and receive passbands with minimal insertion loss while providing required isolation between transmit and receive signals.